In recent years torsional hinged high frequency mirrors (and especially resonant high frequency mirrors) have made significant inroads as a replacement for spinning polygon mirrors as the drive engine for laser printers. These torsional hinged high speed resonant mirrors are less expensive and require less energy or drive power than the earlier polygon mirrors.
As a result of the observed advantages of using the torsional hinged mirrors in high speed printers, interest has developed concerning the possibility of also using a similar mirror system for video displays that are generated by scan lines on a display surface.
Existing CRT (cathode ray tube) video systems for displaying such scan-line signals use a low frequency positioning circuit, which synchronizes the display frame rate with an incoming video signal, and a high frequency drive circuit, which generates the individual image lines (scan lines) of the video. In the existing systems, the high frequency circuit operates at a frequency that is an even multiple of the frequency of the low speed circuit and this relationship simplifies the task of synchronization.
Therefore, it would appear that a very simple corresponding torsional hinged mirror system would use a first high speed scanning mirror to generate scan lines and a second slower torsional hinged mirror to provide the orthogonal motion necessary to position or space the scan lines to produce a raster “scan” similar to the raster scan of the electron beam of a CRT. Unfortunately, the problem is more complex than that. The scanning motion of a high speed resonant scanning mirror cannot simply be selected to have a frequency that is an even multiple of the positioning motion of the low frequency mirror. Although a raster scan CRT system is easily controlled, and the quality is good enough and sufficiently bright for most applications, the display of a corresponding raster scan mirror based system may be dim, and would benefit from an increase in brightness and quality. For example, the scanning light beam is typically on for no more than 10 to 20% of the time. More specifically, the modulated light source is turned on and produces a scan line only when the high speed scanning mirror is moving or sweeping in one direction, or 50% of the time. Likewise, an image frame is generated only when the low speed cyclic positioning mirror is moving in one direction. Consequently, the time is reduced another 50%, thereby leaving a maximum possible time of 25%. Finally, since the high speed scanning mirror travels in one direction, stops, and turns around and then travels in the opposite direction, these turn-around portions (or peak points of the sinusoidal movement) are unsuitable for displaying image pixels. For example, if the high speed resonant scanning mirror has a constant frequency of 20 kHz, yet must slow down, come to a complete stop, and then accelerate in the opposite direction each time the beam sweeps across a display, it will be appreciated that the angular velocity of the mirror movement is anything but constant. However, to generate an image from periodically received pixels, the velocity and movement of the mirror should be substantially linear. Consequently, another 5 to 15% of the mirror movement that is located at turn around or peak portions are not used, which leaves only between about 10 to 20% of the total time that the modulated light beam is generating an image.
Based on the foregoing discussion, an immediate solution to the brightness and quality problem would appear to only require the system to generate an image frame during the unused half of the cyclic motion of one of either the positioning mirror or resonant mirror to double the brightness. Alternately, the unused half of both of the mirrors could be used to increase the brightness by a factor of four. According to the present invention, the image quality is improved and brightness is doubled by using both the positive going and the negative going portions of the slow speed positioning mirror to generate an image. Unfortunately, the problem is not solved by simply deciding to generate an image in both portions of the cyclic motion. The difficulty is aligning the two images for an acceptable display.
However, in addition to aligning the two images, the positioning motion of the low frequency mirror and consequently the low frequency drive signal must also be tied to the image frame rate of the incoming video signals to avoid noticeable jumps or jitter in the display. The term image “frame” as used herein means the image generated by scan lines during the travel of the slow speed mirror in a single direction, and in some embodiments the image frame may not be a full and complete image. For example, in interlaced displays, all of the image lines may not be present. In these embodiments the missing image lines may be filled in on the return sweep. At the same time, however, the high frequency mirror must run or oscillate at substantially its resonant frequency, since driving a high-Q mirror at a frequency only slightly different than the resonant frequency will result in a significant decrease in the amplitude of the beam sweep (i.e. reduce the beam envelope). This would cause a significant and unacceptable compression of the image on the display. Therefore, the high speed mirror drive is decoupled from the low speed mirror drive. That is, as mentioned above, the high speed drive signal cannot simply be selected to be an even multiple of the low speed drive signal.
Further, in a video display, each frame of incoming video signals representing video pixels (such as might be received from a TV station, a DVD player or a VCR player) must still be faithfully reproduced. This means, each pixel of each successive frame of video must be properly located on the screen of the display if distortions are to be avoided. Also of course, if complete image frames are lost or dropped, glitches or artifacts in the display would clearly be observed. Therefore, as described above in a torsional hinged mirror based video system, the low frequency mirror drive must still be synchronized to the flow rate of the incoming video signals. At the same time, however, the high speed mirror must still oscillate at substantially its resonant frequency. The problems discussed above are even further complicated if there has been some degradation of the video signal. For example, if the source of the video signals is a VCR, one common problem such as stretching of the VCR tape could vary the incoming frame rate, which must also be dealt with. Additionally, tracking or synchronizing the low speed mirror and the frame rate should be done in a way that minimizes transients from discontinuities in the drive waveform.
Therefore, a mirror based video system having increased brightness and that overcomes the above mentioned problems would be advantageous, but doubling the beam on time includes many difficult challenges.